The present disclosure relates generally to the manufacture of turbine wheels, and more particularly relates to the casting of turbine wheels.
Turbine wheels in turbomachinery (e.g., gas turbine engines, turbochargers, and the like) operate in extremely challenging environments. The high temperature of the gases passing through the wheel, combined with the high rotational speeds typically experienced, result in severe testing of the strength and/or fatigue-resistance limits of the material from which the wheel is made. At the speeds and temperatures reached by turbocharger turbine wheels, for instance, the strength limit of the wheel material becomes crucial for durability and safety. Turbo shaft speed can sometimes climb to over 200,000 rpm for smaller units, and even the largest turbochargers can reach 90,000 rpm. Turbine wheels can reach 1800° F. (980° C.) and higher in typical turbocharged vehicles, and in top-level motorsports such as WRC they can regularly get up to 1950° F. (1050° C.). The centrifugal stress that the wheel must resist is proportional to the rotational speed squared, and the strength of typical wheels falls off drastically at temperatures above their qualified limits. Wheels are designed to resist these stresses at high temperatures but there is always a limit; a combination of high speed and high temperature increases the possibility of a wheel burst.
There are two basic types of wheel burst: blade and hub. A blade burst occurs when the centrifugal force at speed acting to pull the blades off of the central hub overcomes the mechanical strength of the root sections connecting the individual blades to the hub. Under these conditions if a blade root is too weak it could leave the hub. Hub burst, on the other hand, is the case wherein the main hub that the blades are attached to reaches its ultimate strength limit and breaks into two, three or more large pieces through the centerline of the wheel. The hub is more compact than the blades and is a continuous mass, therefore stronger than the root of each thin blade. However, the hub centerline is at the rotational center-line of the wheel, meaning that the internal stresses are at their maximum at the hub's core. The hub can actually burst at extreme speeds and temperatures.
It has been understood by those working in the turbine wheel field that a fine equiaxed grain structure in the wheel hub is beneficial for reducing the likelihood of hub burst under extreme conditions. Accordingly, various fine-grain casting processes for turbine wheels have been developed.